The application of genetic engineering techniques to plants promises to revolutionize plant agriculture. One result of this revolution would be the ability to control gene expression in plants. For example, pests and diseases have been controlled by applying pesticides or biocides to crop plants; however, application of chemicals to plants affects more than the plant pests or diseases whose control is desired, and poses a general risk to the environment, often with deleterious consequences. In an effort to ameliorate these risks, transgenic plants have been recently developed to constitutively express insect resistance genes or disease resistance genes. However, constitutive expression means that these resistance genes are always expressed, not just when and where and to what level they are needed; such general expression can represent a metabolic drain on a plant, with consequent decreased productivity, or it can be too low to be effective. Moreover, constitutive expression of a protein may not be desirable, particularly if this protein interferes with the early stages of plant development. In other instances, high protein levels cause toxicity to the plant.
Thus, in the last few years, there has been increasing interest among plant scientists on means to precisely control the location, timing and level of expression of transgenes in plants, as well as of endogenous genes. Controlling the location of gene expression, or a more precise control of transgene expression in a specific plant tissue, has been accomplished by means of using one of several different tissue-specific promoters. Controlling the timing and level of gene expression, or temporal and quantitative control of expression, could be accomplished by inducing gene expression upon the application of a specific stimulus. Such inducible gene expression would have numerous practical applications; for example, turning on engineered plant defense genes only upon attack of a pathogen or insect predator could save millions of dollars in pesticide application, as well as decrease unwanted adverse environmental effects. Inducing gene expression would also be a powerful research tool, where it could be used in studies ranging from examining phenotypes associated with specific gene expression to investigations of gene interactions in plants.
One type of inducible promoter is a chemically inducible promoters. These are synthetic promoter systems often constructed by combining known regulator elements whose activity is modulated by the presence of chemical effectors.
A chemically inducible promoter preferably satisfies several criteria to be useful in an agricultural application. Such criteria include sufficient stability and relative non-toxicity of chemical inducers. Chemically-induced gene promoters have been isolated, but none of these are suitable for practical application because of the nature of the chemical inducers. The chemicals are either too volatile (such as ethanol) or toxic to the environment or the plants to which it is applied (such as copper ions, antibiotics, or steroids).
Thus, what is needed are inducible promoters that are activated by chemicals which are sufficiently stable and non-toxic to the environment, including the plants to which they are applied. Preferably such chemical inducers can also be easily applied to large acreages of crop plants; even more preferably, such chemical inducers are inexpensive.